Multi-speed vehicular transmissions, particularly those using planetary gear arrangements, require a hydraulic system to provide controlled engagement and disengagement, on a desired schedule, of the clutches and brakes or torque transmitting mechanisms that operate to establish the ratios within the planetary gear arrangement.
These control systems have evolved from substantially pure hydraulic control systems, wherein hydraulic devices produce all of the control signals to electro-hydraulic control systems, wherein an electronic control unit produces a number of the control signals. An electronic control unit emits electrical control signals to solenoid valves, which then issue controlled hydraulic signals to the various operative valves within the transmission control.
With many of the early pure hydraulic and first generation electro-hydraulic control systems, the power transmission utilized a number of freewheel or one-way devices which smooth the shifting or ratio interchange of the transmission during both upshifting and downshifting of the transmission. This relieves the hydraulic control system from providing for the control of overlap between the torque transmitting mechanism that was coming on and the torque transmitting mechanism that was going off. If this overlap is excessive, the driver feels a shudder in the drivetrain, and if the overlap is too little, the driver experiences engine flare or a sense of coasting. The freewheel device prevents this feeling by quickly engaging when the torque imposed thereon is reversed from a freewheeling state to a transmitting state.
The advent of electro-hydraulic devices gave rise to what is known as clutch-to-clutch shift arrangements to reduce the complexity of the transmission and its control system. These electro-hydraulic control systems are generally perceived to reduce cost and reduce the space required for the control system.
In addition, with the advent of more sophisticated control systems, the transmissions have advanced from two-speed or three-speed transmissions to five-speed and six-speed transmissions. In at least one presently available six-speed transmission, just five friction devices are employed to provide six forward speeds, neutral condition, and a reverse speed. The torque capacity of a torque transmitting mechanism (on-coming or off-going) involved in a shift may be controlled by the combination of an electrically activated solenoid valve and a pressure regulator valve or trim valve. In a typical system, the solenoid valve is activated by pulse-width-modulation (PWM) at a controlled duty cycle to develop a pilot or control pressure for the pressure regulator valve or trim valve, which in turn, supplies fluid pressure to the torque transmitting mechanisms in proportion to the solenoid duty cycle.